Application of alkaline slag and phosphogypsum for alleviating soil acidity in an Ultisol profile: a short-term leaching experiment

Industrial by-products are being explored as alternatives to lime and gypsum for correcting soil acidity in top- and sub-soil because they are cheap and easy to obtain. In this study, the ameliorating effect of alkaline slag (AS) on soil acidity in an Ultisol profile was compared with lime (CaCO3) and phosphogypsum (PG). A column leaching experiment was used, and the leaching columns received a total of 1,632 mm of simulated rainfall which was divided into ten events over 2 months. Results indicated that lime increased soil pH and exchangeable calcium (Ca2+) in top- and sub-soil of 0-20 cm but did not affect soil acidity of layers below 20 cm. AS had a similarly positive effect to lime in increasing soil pH and decreasing exchangeable acidity in top- and sub-soil but increased total amount of soil exchangeable Ca2+ and magnesium (Mg2+) throughout the 40-cm profile. Surface application of AS had a comparable effect to PG, lime + PG, and AS + PG in increasing soil exchangeable Ca2+ in the sub-soil. Moreover, AS was more effective than lime and PG in enriching Mg in the Ultisol. Lime + PG and AS + PG had similar effects in alleviating soil acidity throughout the soil profile. Sulfate (SO4 (2-)) in the PG and SO4 (2-) and chloride (Cl-) in AS, as accompanying anions, promoted the migration of Ca2+ or Mg2+ from the top- to sub-soil and induced their ameliorating effects on sub-soil acidity. AS can play the same role as lime + PG in correcting soil acidity in the top- and sub-soil in this Ultisol, since AS contains both calcium carbonate and calcium sulfate.